This document discusses the composition and types of white blood cells (leukocytes). It explains that leukocytes are formed partially in the bone marrow and lymph tissue and transported via blood to different parts of the body. There are two main types of leukocytes - granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lymphocytes, monocytes). The document provides details on the characteristics, functions and life spans of these different types of white blood cells. It emphasizes that leukocytes are mobile defensive cells that play an important role in the body's immune system.